How to use an ATX PSU outside of a PC

Current Status:   Complete
Date:                    8th March 2004
License:              Creative Commons Attribution 3.0 Unported License/

Added info on testing an ATX supply ,where to obtain the ATX specification from and a quick FAQ.

Moved the circuit to utilise the ATX PSU to a sub-page.

What is the ATX specification and more importantly, how can I use the PSU?

Got problems with an existing ATX PSU?

Want to use an ATX PSU away from a PC?

Hopefully these pages can help you.

ATX Specification and FAQ

What is the ATX specification?

From the ATX spec..

ATX was developed as an evolution of the Baby-AT form-factor and was defined to address four major areas of improvement: enhanced ease-of-use, better support for current and future I/O, better support for current and future processor technology, and reduced total system cost.

ATX combines the best functionality from the form factors that dominated the computer industry in the early 1990s:

the high integration of LPX and the expandability of Baby-AT. The ATX form factor is essentially a Baby AT motherboard rotated 90 degrees within the chassis enclosure and providing a new mounting configuration for the power supply. The processor is relocated away from the expansion slots, allowing them all to hold full length add-in cards. The longer side of the board is used to host more on-board I/O.

What way does the fan in the PSU blow, in or out?

Inwards. It sucks in air from the outside and expels it through the PSU casin to cool the system components, thus removing the need for an extra fan.

Can I use an ATX psu with an AT motherboard?

No. The motherboard connectors are incompatible.

So what is special about the ATX PSU?

  • It uses one high density connector to connect to the motherboard.
  • It supplies 3.3 Volts in addition to the traditional 5 and +/- 12V supplies.
  • The fan in the PSU is used to cool your system's components.

How can I get hold of the ATX specification and how much is it?

It is free and available from the following location at Intel's site,

How can I test a suspect ATX PSU?

See the faultfinding tips below.

ATX Fault finding tips

Here are some tips that may help you in determining a fault with your system.

1) Is the AC mains switch (if fitted) turned on and is the computer turned on at the mains socket?

If no, well do you expect anything to work?
Proceed to step 2.

2) Did the soft start (case switch)  allow you to turn on your computer?

If yes all seems well, double check your connections and make sure they are tight. If no, proceed to step 3.

3) If the PSU turned on, was there a high pitched noise coming from the PSU?

You may have a short circuit somewhere in your wiring, check all connection for damaged wires or a peripheral wired up backwards, then proceed to step 4.

4) Try disconnecting the PSU from the mains and leaving it for 5 minutes to allow the Crowbar safety circuit to reset.

Go back to step 1 and repeat.

5) Disconnect the PSU from the motherboard and try the following:

Connect the PS-ON pin, typically green but check, to 0V, a black wire, now turn the PSU on, the fan should start and you should be able to measure outputs on all the power lines, 3.3, 5, 12 and -12 Volts.

If you are confident, you could put a jumper across the Case switch pins of your motherboard. This would allow you to determine if the case switch actually works, see my ATX circuits for more details.

Typically there is a fuse inside a PSU, that could have gone. If you are unsure about replacing/checking this fuse consult an expert. A switchmode PSU, like an ATX power supply can retain upto 400 V DC for some time after the power was switched off.

I would disconnect the PSU from the mains, then try and turn on the PSU, this would drain any remaining power from the unit. See safety warning above.

If you have got this far and your PSU does not work, it could be faulty. It could be the case/switch that is faulty. Normally it is cheaper to buy another case than a replacement PSU so consider this option.


Updated 27 September 2020